A typical integrated circuit may include millions of devices or sub circuits. Some of the integrated circuits contain a lot of identical sub circuits. For example, a memory chip contains a lot of identical memory cells. Even if only one memory cell has defects, the entire chip may be rendered defective.
To increase yield, redundant memory cells are manufactured on the same chip. If some of the primary memory cells have defects, redundant memory cells can be used to replace the defective primary memory cells. This redundant configuration permits the semiconductor memory device to continue to operate in a normal state. The primary memory cells and redundant memory cells are all connected via fuses controlled by control circuits on the chip. As stated above, if a defective memory cell is discovered, a fuse coupled to the defective memory cells is blown and the redundant memory cell is connected instead. Accordingly, the semiconductor chip with defective primary memory cells can operate normally.
Highly integrated semiconductor memory devices have a fairly high manufacturing cost, which causes a big loss if any defective cells are discovered. This is why the memory devices include redundant memory cells for replacing defective primary memory cells. Types of fuses deployed in such semiconductor memory devices include electrical fuses selectively cut by the flow of excessive current, and laser fuses selectively cut by an applied laser beam. In contemporary systems, laser fuses are widely used due to their simplicity in use and layout. Electrical fuses are commonly used in semiconductor memory devices such as Electrically Erasable Programmable Read Only Memory (EEPROM) while the laser fuses are very often used in Dynamic Random Access Memory (DRAM).
FIG. 1 illustrates a conventional laser fuse structure. A laser fuse 100 is formed close to the top surface of the chip. Through layers of metal islands 112 and vias 104, the laser fuse 100 is connected to two conductive lines 106, which are in turn connected to integrated circuits. By etching and opening a fuse window 118, the laser fuse 100 is exposed and can be broken by a laser, and an open circuit is formed between the conductive lines 106. A redundant circuit will then replace a malfunctioned circuit.
Due to the high stress introduced during the laser process, a protection ring 116, which substantially surrounds the laser fuse 100, is typically formed to protect integrated circuits on the same chip from mechanical stress and/or to terminate any micro crack generated. The protection ring 116 is formed of interconnected metal lines and vias, and extends from the top metallization layer down to metallization layer one. A top view (not shown) will illustrate that the protection ring 116 forms a closed loop surrounding the laser fuse 100.
Mechanical stress and heat generated by laser processes damage the underlying structures and materials, such as low-k dielectric layers that are widely used as inter-metal dielectrics. A copper plate 114 is then used to buffer the mechanical stress. Typically, copper plate 114 connects to one of the metal islands 112. The top dielectric layer 120 is often formed of undoped silicate glass (USG) in order to improve the mechanical property and prevent moisture penetration. Since the dielectric material in which the fuse lines are formed has low thermal conductivity, no good heat path exists. A portion of heat dissipates to underlying metallization layers, which are formed of stacked layers of metal and low-k dielectrics. As a result, high thermal gradient and different thermal expansions occur, causing delaminating between layers.
Therefore, there is the need for a method preventing heat generated by laser processes from peeling the underlying layers.